What causes the movement of calcium and sodium ions in and out of cardiac cells?

Alcium is removed from cells by two basic mechanisms.  The first mechanism involves an ATP-dependent Ca++ pump that actively removes calcium from the cell (see figure at right).  The second mechanism is the sodium-calcium exchanger.  The exact mechanism by which this exchanger works is unclear.  It is known that calcium and sodium can move in either direction across the sarcolemma. 

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